Here Comes The Basil Blues

The gods must be angry with us.  Or, more likely, the increase in world travel and shipment of goods is transporting pests around the globe like never before.  From the many examples I could grouse about, today I’ll alert you to downy mildew of basil.  If you don’t grow basil, or if you don’t love pesto, Margherita pizzas or tomato, cheese and basil sandwiches, then you can skip this trauma and just fret over world events.  But if you’re a basil aficionado like me, then please read on.

Basil downy mildew up close.  Photo by Meg McGrath

Basil downy mildew, a fungus-like disease, had its first U.S. finding in Florida in 2007 and made it here to New York the following year.  We don’t know how much or just when it will show up each season, since it can’t take our cold winters and must be transported here anew.  Like the anticipated return of a rare bird, this seasonal comeback makes folks in the know keep their eyes peeled for its return.  So far this year, first-responder Meg McGrath, a plant pathologist at the Long Island Horticultural Research and Extension Center, reported finding basil downy mildew at a big chain garden center on June 14.  She’s also getting reports of it from home gardeners, and notes in her blog that fellow pathologists have discovered it in several states already.  The growing season is still young, but it appears that 2018 will be tough for the basil business.

Basil downy mildew can be tricky to detect if you aren’t in the know.  The initial symptoms are a yellowing of the foliage from the bottom of the plant upward, a malaise resembling nutritional deficiency.  However, this is a blight a little fertilizer won’t fix.  Lurking on the lower leaf surfaces will be a fuzzy gray growth of mycelia.  As the disease progresses, the leaves will eventually blacken and die.  All of the myriad types of sweet basil (Ocimum basilicum) are susceptible, although some folks have noticed that purple-leaved and lemon types seem a little more resistant.  Only varieties of O. americanum, a species primarily grown for medicinal purposes, have shown no symptoms or sporulation when inoculated with basil downy mildew. Perhaps some creative plant breeder can transfer this resistance to sweet basil and make a small fortune.  As the disease doesn’t harm people per se, it is not dangerous to ingest a few infected leaves, but I can’t imagine it would be an appetizing experience, either.

Lower leaves  first turn yellow then brown.
Photo by Meg McGrath

It is difficult to outsmart basil downy mildew.  If your plants bought from the garden center look fine, the problem may blow in on the wind.  If you start your plants from seed, you aren’t home-free either necessarily, because this pathogen (unlike many others) can be carried by seeds.  If your basil eventually becomes blighted, bag it and trash it immediately.  Keeping plants well-spaced and watering early in the day so the foliage is kept as dry as possible are preventative measures.  Fungicides can be used, but who wants to eat basil sprayed with fungicide?  That doesn’t taste like summer.    

For more information, check out the very detailed website provided by Cornell Plant Pathologist Margaret Tuttle McGrath's  website.

Text by David Chinery, Horticulture Educator for Cornell Cooperative Extension of Rensselaer County

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Previously published in Root Concerns - July, 2014 

Root Concerns is a gardening newsletter produced by the Cornell Cooperative Extensions of Rensselaer, Albany, and Schenectady Counties. Because its contents have not been previously indexed by Internet search engines, relevant articles from past issues will be occasionally reprinted.